Plot summary
Security chief Odo places El-Aurian Martus Mazur in a holding cell after accusing him of swindling aboard Deep Space Nine. While in his cell, Martus acquires a gambling device that alters the laws of probability after his cellmate, an old man named Cos, dies. His luck changes and he is released from confinement after the elderly couple he conned decide not to press charges.
With his new and intriguing device, he sets up a bar and casino with larger versions of the original device as gambling machines. The bar is a success and draws business away from Quark's bar. The device soon turns on him and he himself is conned into losing a large sum of money. In addition, the couple he swindled out of money change their mind and press charges against him.
Meanwhile, Miles O'Brien attempts to get in shape so that he can compete with Julian Bashir at racquetball. As part of a ploy to regain lost business for the bar, Quark is able to trick O'Brien and Bashir into participating in a public tournament that people can bet on. When O'Brien performs abnormally well, he realizes that there are statistical abnormalities on the station. The crew discover the effects of the devices in Martus's establishment, and O'Brien and Bashir agree to cancel the rest of the game. To prevent more tampering with natural probability, Commander Sisko and science officer Dax destroy the device.
Credits
Adapted from Wikipedia contributors, Rivals (Star Trek: Deep Space Nine). Paragraphs and formatting adjusted; illustrations, reference markers, block quotations and non-story material omitted.